System for automatic oscillation of an apron tip

ABSTRACT

A hydraulic control system for an articulated scraper apron having an articulated apron tip which provides automatic oscillation to the apron tip and thereby facilitates the closing of the apron through material which would otherwise prevent the apron from closing completely.

United States Patent 1191 Hicks et a1.

[54] SYSTEM FOR AUTOMATIC OSCILLATION OF AN APRON TIP [75] Inventors: Leon E. Hicks, Joliet; Henry J. Jessen, Wilmington, both of I11.

[73] Assignee: Caterpillar Tractor (10., Peoria, 111. [22] Filed: Dec. 14, 1970 [21] Appl. No.: 97,779

52 us. (:1. 37/126 AD, 37/129 51 Int. Cl. E021 9/20 58 Field 61 Search 37/129, 141, 126 A,

37/126 R, 126 AA, 126 AD [5 6] References Cited UNITED STATES PATENTS 4/1971 Peterson 37/129 X 1/1966 Spannhake 37/141 R [111 3,739,505 June 19, 1973 3,145,488 8/1964 French 37/141 R 3,269,039 8/1966 Bodine 37/141 R X 2,946,144 7/1960 Anderson 37/129 2,335,231 11/1943 Armington et al..... 37/129 3,181,431 5/1965 l-lein et a1. 91/414 3,473,242 10/1969 Martin 37/141 R X 3,443,327 5/1969 Martin A 117/14] R X 2,562,193 7/1951 Johnson 37/129 Primary ExaminerEdgar S. Burr Att0rneyFryer, Tjensvold, Feix, Phillips & Lempio [57] ABSTRACT A hydraulic control system for an articulated scraper apron having an articulated apron tip which provides automatic oscillation to the apron tip and thereby facilitates the closing of the apron through material which would otherwise prevent the apron from closing com pletely.

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AT TORN E YS PATENTED JUN! 75 INVENTORS LEON E. HICKS HENRY J. JESSEN ATTORNEYS SYSTEM FOR AUTOMATIC OSCILLATION OF AN APRON TIP CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS Pertinent cross references are US Pat. Nos. 3,181,431, and 3,115,716, both assigned to the assignee of this invention. Cross reference is made also to Application Ser. No. 196,817, filed Nov. 8, 1971 and also assigned to the assignee of this invention.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates generally to scraper aprons and more particularly to structure for automatically and continuously pivoting the tip portion of the apron in an oscillating or reciprocating manner and thereby facili tating the closing of the apron through material which would otherwise prevent the apron from closing completely.

The inability of a scraper apron to close efficiently and completely when handling most materials is an old and persistent problem. Even with a hydraulically actuated apron, closure is generally slow and eratic when loading some soils. In loose, granular material, the usual result of this difficulty is a substantial leakage of material out of the scraper bowl before the apron can be fully closed. A conscientious and experienced operator can minimize this loss by frequent movements of the bowl lift and apron close controls. In many cases, however, an operator will merely raise the bowl, overrun the bulldozed pile, and then close the apron without giving heed to the lost portion of the load.

In dry, granular material, little relative motion takes place between the soil and the apron surface. The material quickly fills any voids and is squeezed by attempted closure of the apron. Any further closure of the apron requires a very high actuating force, producing full sliding of the apron surface around the densely packed soil in the bowl. The internal closing resistance builds up very rapidly with apron movement, and the uniform consistency (lack a voids) in the soil contributes to the rapid lockup of the apron. In a material which is cohesive and possesses void spaces, greater apron closure can be effected before a very high resistance is developed.

Three of the main reasons why an apron will not close in dry, sandy, or rocky material are as follows:

1. friction resistance forces on the inside of the apron are large enough to prevent adequate closing;

2. most aprons are relatively thick box sections which causes compression of the material ahead of such sections rather than the desirable knifing of such sections through the material, and, friction forces on the outer surface of the apron in contact with the bulldozed material additionally contribute to the inadequate closing and the wide box sections while the friction forces on the outer surface of the apron are enough, in and of themselves, to prevent closing of the apron; and

3. a rock or rigid object, between the lip on the apron tip and the cutting edge, will prevent closing.

in addition, loose, granular material is often lost from the scraper bowl since the material tends to spill out below the apron before it is closed. This occurs if the apron does not close at a fast enough rate. Furthermore, if the apron is prevented from fully closing by means of a rock or other similar object, a great deal of such loose material can be lost.

Since many scrapers are purchased for use in loose, granular, or rocky, material where apron spillage can be a significant problem, the present invention has been developed so that the apron tip may be automatically and continuously reciprocated or pivoted fore and aft to work its way through the material ahead of the cutting edge.

It is therefore an object of this invention to provide a scraper apron control system which produces an improved operation when handling loose, granular, or rocky material.

It is also an object of this invention to provide a control system for a scraper apron wherein apron spillage of such materials is significantly reduced.

It is a further object of this invention to provide a control system for a scraper apron with means to automatically and continuously reciprocate or pivot the apron tip in fore and aft motion, thus allowing the tip to work its way through the material ahead of the cutting edge.

Other objects and advantages of the present invention will become apparent from the following description and claims as illustrated in the accompanying drawings, which, by way of illustration only, show preferred embodiments of the present invention and the principles of operation thereof. It is to be understood that the scope of the invention is not to be limited thereto, but is to be defined by the scope of the ap pended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of the righthand portion of the control system showing several of the system valves in cross section;

FIG. 2 is a schematic view of the lefthand portion of the control system shown attached to an articulated apron and apron tip and showing the oscillation control valve in cross section;

FIG. 3 illustrates how the righthand and the lefthand portions of the control system shown in FIGS. 1 and 2, respectively, are to be joined together to produce the complete system;

FIG. 4 contains cross sectional views of the apron sequence and relief valve and special relief valve which are schematically shown in FIG. 1; and

FIG. 5 shows in cross section an alternate embodiment of the oscillation control valve of FIG. 2.

DETAILED DESCRIPTION Referring now to FIGS. 1 and 2 there is shown a hydraulic control system for operating a scraper apron and articulated apron tip. The system comprises main control valve 10 which fluidly communicates hydraulic fluid under pressure from pump 11 to the scraper apron system shown generally at 12 by means of relief valve 20, special relief valve 30, sequence and relief valve 40, and an oscillation control valve 50.

The scraper apron system is structured as follows. An apron tip 13 is pivotally secured to the arms of apron 14 by means ofa plurality of hinges 15 extending across the front of the apron. Pivoting of the tip is accomplished by a plurality of hydraulic jacks 16 secured to the outer ends of the front surface of the arms by pivots 17 on brackets 18 projecting from the arms and to the apron tip by means of pivots l9.

Retraction of the jacks 16 causes the tip to pivot forward and away from the scraper cutting edge to a relatively steep angle with respect to the material ahead of the bowl, allowing the tip to knife its way through granular or rocky material which might otherwise cause difficulty in closing the apron.

The entire apron system may be raised and lowered by means of a piston-cylinder 21 acting upon a lever 23 which is pivotally attached to the gooseneck 25 and a link 27 which is pivoted on the main arcuate portion of the apron. Retraction of the piston into the cylinder powers the apron system downwardly in an are about pivots 29 on scraper bowl 31. If needed, a more complete description of the operation of the scraper apron system may be had by referring to cross referenced U.S. Pat. Application Ser. No. 196,817.

Referring now to FIG. 1 there is shown in partial cross section the main control valve or means. The main control valve or means has three control stems and includes a sequence valve or means hydraulically linked thereto. If needed, a more detailed description of the structure and function of main control valve 10 may be had by referring to cross referenced U.S. Pat. Nos. 3,115,716 and 3,181,431. Bowl cylinder stem 33, which is linked to manual bowl lever 35, controls the action of the apron bowl cylinders 37. The apron cylinder stem 39 which is linked to manual apron lever 41 controls the action of the piston-cylinders 21 in order to raise or lower the apron. An ejector stem (not shown) which is linked to ejector lever 43 serves to control the operation of an ejector (not shown). The ejector is not pertinent to this discussion and therefore no further reference will be made to its operation.

Pump 11 supplies the hydraulic fluid under pressure which is necessary for the actuation of the hydraulic circuits. When the aforementioned stems in main control valve 10 are in the neutral position, fluid flowing from the pump will be communicated to the valve by line 45 and returned to tank via line 47. When the bowl stem 33 is moved to the raise position, pump flow will be blocked from the tank line and fluid in line 45 will be directed to a line 26.

The line 26 connects with a sequence valve or means 20 and is communicated to a chamber 28 therein. Fluid in chamber 28 will flow through an orifice 32 contained in spool 34, into chamber 36, passage 38, chamber 22, passage 42, and a line 44 to tank. Flow through the orifice 32 will cause a pressure drop across a spool 34 and will open the spool against the biasing force of a spring 46 to connect the chamber 28 with a line 48. Line 48 connects with the rod ends of the bowl cylinders, one of which is shown at 37, to raise the bowl by means of a mechanical connection with the bowl (not shown). Fluid being expelled from the bowl cylinders into a line 51 is discharged to tank via tank line 47 by internal passages in main control valve 10.

When bowl stem 33 is shifted to the lower position, flow in line 45 is directed to the line 51 by internal passages in main control valve 10. Similarly, the line 26 is communicated with a tank passage within the control valve 10. Fluid being directed to the bowl cylinders 37, through the line 51, is not required to be at a very great pressure since the weight of the bowl will assist in its lowering.

Fluid being expelled from cylinders 37 to line 48 will cause a build-up of pressure in the chamber 52 that will work against the differential area of spool 34 and will open the spool so that the chamber 52 will be connected to chamber 28, and thence to line 26 so that fluid can then be expelled to tank through the control valve 10 and tank line 47. However, when the cutting edge 24 of the bowl strikes the ground, pressure will be available in line 51 from the pump to apply downward pressure for a cut.

When the apron is to be raised, the apron stem 39 will be shifted so as to connect fluid in line 45 with a line 54 by internal means within control valve 10. Fluid in line 54 is directed to the head end of the apron cylinders 18 by means of line 55 as best seen in FIG. 2. Fluid being expelled from the cylinders is directed to a line 56 which connects with oscillation control valve or means 50 by way of line 57.

The special relief valve or means 30 prevents pressure in line 56 from communicating with pilot line 59 in order to prevent activation of the apron sequence and relief valve 40. Reference may be had to previously mentioned U.S. Pat.Nos. 3,115,716 and 3,181,431, for more detailed description of the operation of the sequence and relief valve.

Referring now to FIG. 4, pressure in line 54 is also directed through a line 61 to a chamber 62 of valve 40 and thence through a passage 64, annulus 65, passages 66, 67, and into chamber 68. Force created by the hydraulic pressure will move a piston to the left, as viewed in the drawings, to open a load check valve 72 to connect passage 74 to a passage 76 by way of annulus 75. The passage 74 is communicated with a line 78 which, in turn, is connected to the line 56 through valve 30. Fluid is thus allowed to be expelled from cylinders 21 into line 57 and then line 56 to be directed to tank, since the passage 76 is connected to a line 80 which directs fluid to control valve 10 and then to tank line 47.

When the bowl is being raised, pressure will be developed in the rod ends of apron cylinders 21 because of the geometry of the arms and linkages. The pressure thus developed is relieved in the following manner. Pressure in the cylinders is directed through lines 56, 57 and thence to chamber 74 of valve 40 by way of valve 30 and line 78. Passage 74 is connected to a passage 82 and a chamber 84. Pressure in the chamber will act upon a piston 86 which moves it and abutting spool 88 to the right to connect passages 74 and 81 with chamber 68 by means of annuli 89, 91 and passages 64 and 66, 67 by means of annulus 65. Pressure in chamber 68 will shift piston 70 and load check 72, as described hereinabove, in order to connect passage 74 to passage 76 by way of passage 81 in order to relieve the rod ends of the apron cylinders to tank.

When the apron is to be lowered or closed, the apron stem 39 will be moved to the lower position by means of lever 41. As seen in FIG. 1, the end of control lever 41 is tiltable about pivot 97 and is linked to an air valve or means 103 by means of link 99 and pivots l0] and 107. The linkage enables the tilting of the end of control lever 41 to actuate air valve 103 which communicates with the source of air pressure (not shown) by way of'line 105. Valve 103 is spring biased in the closed position, however, when the end of the control lever is tilted and the valve is opened, air pressure will be communicated through the valve to line 90. Pressurization of line will do two things.

First of all, it will activate an actuator 92 in order that the bow] stem 33 will actually be moved so as to connect line 45 with line 26. Secondly, pressurization of line 90 will direct air pressure to valve and will work against spool 93 and will move that spool leftwardly against the biasing force of spring 94. The movement of the spool thus described will cause chamber 28 to be communicated with line 96 by way of an annular groove 98 around spool 93 and by way of passage 100. Movement of the spool additionally blocks passage 38, thereby blocking chamber'36 from tank in order to transform spool 34 into a relief dump spool regulated by the spring biased poppet valve 109. Since the bowl stem will have been moved to communicate supply fluid to line 26 and thence to line 96, fluid will be directed to valve 30 by line 96.

Referring now to FIG. 4, when line 96 is pressurized, the pressure in pilot line 59 will cause piston 111 to move to the left and thus set the spool 88 of valve 40 such that it cannot shift to the right to direct pressure into chamber 62 and thence to line 61. Line 96 communicates with chamber 102 in valve 30 and will thereby open check valve 104 against the bias of its spring 113. This will allow fluid from line 96 to be directed to line 56 which will, in turn, direct fluid to the rod end of the apron cylinder 21 via line 57 and thereby start the closing action.

Referring now to FIG. 2, line 56 is also connected to valve 50 by way of line 57. Line 57 communicates with a passage 110 within valve 50. The passage 110 communicates with a line 112 by way of an annular groove 114 around a spool 116 which is reciprocably mounted within valve 50. Pressure will thus be applied to the head end of cylinder 16 which controls the action of the apron tip 13.

When spool 120, also reciprocably mounted in valve 50 and rightwardly biased by spring 121, is in the position shown in the figure it will block a passage 122 and a line 124 which connects to the rod end of cylinder 16.

If the operator wishes to have cylinder 16 automatically and continuously oscillate back-and-forth or re ciprocate so as to aid closing of the apron, he would merely tilt the end of control handle 41, shown in FIG. 1, to the left. As aforementioned, this will cause the air pressurization ofline 90 which causes such air pressure to be directed to the head end of a piston 119 which abuts the right end of reciprocably mounted spool 120 in control valve 50. The pressure thus communicated will cause a shifting of the spool to the left so that passage 122 is opened to communication with passage 128 by way of annular grooves 126 and 127. Fluid pressure will be directed from passage 128 by way of annular grooves 132 and 133 to passage 130 and thence to line 55. Fluid pressure is then communicated from line 55 through line 54 to tank line 47 by way of internal passages in control valve 10. Thus, fluid is allowed to be expelled from the rod end of cylinder 16 and the piston and rod assembly 135 will be extended. A drain passage 129 and line 131 are provided to drain leaked fluid to tank.

If the apron tip 13 is in any position other than the closed position when the control handle is moved to neutral, it will return to the closed position as follows: Venting of valve 90 (neutral position of control handle) will allow spool 120 to shift to the right and will block annulus 126 from passage 128. Back pressure in passage 128 will then build to a point in chamber 140 to shift spool 147 to the right to direct flow from line 56, thru annulus 114 into line 112 to start to close apron tip 13 by directing fluid to the head end of cylinder 16. Fluid pressure communicated from the rod end of cylinder 16 will build in line 124 and work against check valve 123 communicating line 124, and chamber 122 with annulus 127, passage 128 to tank line 54 so that the tip 13 can be closed. The above action is auto matic and assures that the tip 13 will return to a closed position no matter what position it is in when the control handle is moved to neutral.

When the piston and rod assembly 135 reaches the end of its stroke, the pressure build-up will be communicated to passage 112 by way ofline 57. Also, pressure build-up in annular groove 136 around spool will be communicated by a line 137 to a chamber 139 and will work against a piston 138 which abuts the end spool 116. The pressure thus communicated will shift the spool to the second of its two operating positions and will be held in place by spring biased ball detents 141, 143. The shifting will occur when the pressure reaches a predetermined level pursuant to the spring constant which has been previously selected.

When the shifting of the spool has been accom plished, flow will be blocked from passage 110 to line 112 by land 145 on spool 116. However, passage 110 will then be connected to passage 128 via annular groove 147 around spool 116. Thus, fluid will be directed'from passage 128 to line 124 and will thence enter the rod end of cylinder 16 which will start to shift the piston and rod assembly to the retracted position. When it reaches its furthest point of retraction, a buildup of pressure in passage 128 will be directed to a chamber 140 on the other end of spool 116 by way of a passage 142.

A predetermined pressure in chamber 140 will work against a piston 144 abutting the end of spool 116 and will cause it to move back to the first of its two detented positions. This will again connect passage 110 with line 112 and will reverse the movement of piston and rod assembly 135. The automatic, continuous oscillating or reciprocating action thus produced will continue as long as the operator maintains control handle 41 in its leftwardly tilted position.

During the oscillating mode of operation thus described, the apron assembly 12 will have been moving in a downward direction due to fluid entering the rod end of cylinder 21. When the cylinder and piston rod assembly 146 has been moved to its completely retracted position, build-up of pressure in line 57 will be communicated to line 56. As best seen in FIG. 1, this pressure build-up will be communicated to chamber 28 by way of valve 30, line 96, annular groove 98, and pas sage 100. The pressure build-up in chamber 28 will pass through orifice 32 and will work against poppettype check valve 109. When the pressure reaches a predetermined limit, it will cause the check valve to open which will in turn allow flow through orifice 32 to open spool 34. Thus, fluid being supplied to line 26 from pump 11 will be directed to bowl lift cylinders 37 by way of line 48.

By the preceding description, it has been demonstrated how a single handle 41 can control the actions of a scraper by first closing the apron and then lifting the bowl when the scraper is loaded. The sequence type of operation will be performed by valve 20 to first close the apron and, if necessary, the operator may oscillate the apron tip back-and-forth while the apron is closing. Once the apron is fully closed, the [bowl cylinders will raise the bowl.

An alternate embodiment 50 of valve 50 is shown in FIG. 5. It is to be parenthetically noted that a prime will be added to the identifying numbers in the following discussion to denote elements which have correspondingly numbered parts in the primary embodiment. Spool 116 is moved to one of two detented operating positions by applying pressure to either one of pistons 144 or 138' in a manner similar to that described for the primary embodiment. The difference with this embodiment is that the pressure fluid must first pass through a trigger valve or means 150 or 152, depending upon the position of spool 116. The biasing springs of the spools of the trigger valves can be accurately shimmed such that an exact pressure is required to shift the trigger valve spools 151 or 153 in order to direct pressure to either of chambers 140' or 139 behind pistons 144 and 138, respectively.

For example, a stack of shims 154 are shown in association with spring 155 which biases spool 151. No shims are shown associated with spring 156 which biases spool 153 in order to illustrate the fact that shims may not be required.

This embodiment basically differs from the prior embodiment in that with the prior embodiment the only resisting force against movement of spool 116 was detents 141, 143. Thus, the force from fluid pressure required to shift spool 116 would necessarily cover a wide range. This could cause premature shifting of direction of the oscillating tip cylinder.

The operation of valve 50' and the trigger valves is as follows. Fluid pressure from line 55' is directed by way of passage 130, annular grooves 133 and 132, passage 128 and line 124 to cylinder 16'. When rod 135 in cylinder 16' is fully retracted, pressure build-up in line 124' will be communicated by way of line 157, and passage 158 to chamber 159. The increased pressure in chamber 159 will cause piston 160 to move spool 151 against the biasing force of spring 155. Thus, line 157 will be brought into fluid communication with line 160 and thence chamber 140 by way of annular groove 171 around spool 151.

Pressure in chamber 140' will cause piston 144' to move spool 116 to its rightmost detented position where it is held by ball detents 141, 143 as shown in the figure.

When it is desired that cylinder 16 be oscillated automatically, line 90' would be pressurized in the manner previously discussed with the primary embodiment. This would cause spool 119 to be shifted leftwardly and cause spool 120 to move against its biasing spring 121'. This would, in turn, establish fluid communication between line 57 and line 161 by way of passage 110', groove 114', passage 162, and passage 163. The fluid pressure would be communicated from line 161 to chamber 165 in trigger valve 152 by way of passage 164. The pressure thus communicated would cause piston 166 to move abutting spool 153 against the force ofits biasing spring 156 and thereby open up fluid communication between line 161 and line 167 by way of annular groove 170 around spool 153. Fluid thus communicated passes into chamber 139 where it acts upon piston 138 and causes the piston to move spool 116' to the left into its leftmost detented position. As with the primary embodiment suitable drain passages, e.g., 168, 169, and 129 and drain lines, e.g., 171, 131, are

provided whereby leaked pressure fluid may be drained to tank.

It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention and that the scope of the invention is not to be limited thereto, but is to be determined by the scope of the appended claims.

What is claimed is:

1. In a scraper bowl having an apron pivotally mounted thereon, an apron tip pivotally mounted on the apron by an apron tip pivot means, hydraulic cylinder means for actuating the bowl, apron and tip, a source of fluid pressure, and main control valve means for directing fluid from the source of fluid pressure to the hydraulic cylinder means,

the improvement comprising,

oscillation control valve means for oscillating the apron tip in fore-and-aft arcuate motion about said apron tip pivot means, wherein the hydraulic cylinder means comprises a bowl cylinder, an apron cylinder, and a tip cylinder having head and rod ends, wherein the oscillation control valve means comprises a main control valve for actuating an oscillation control valve, and wherein the oscillation control valve comprises a body with a first and a second reciprocably mounted spool located therein, said first spool serving to communicate pressure fluid from said main control valve to either the head or the rod ends of the tip cylinder depending upon the position of said first spool, said second spool being biased to a first position by spring means at one end of said spool and being shiftable to a second position by means of a pressure fluid directed by said valve means for actuating said oscillation control valve, said second spool serving to direct pressure fluid to said first spool whereby said first spool is reciprocated thereby causing the tip cylinder to oscillate in a fore-and-aft manner.

2. The invention of claim 1 wherein a first and second spool pistons are additionally mounted within said body and abutting each end of said first spool and wherein pressure fluid is alternatively directed from said source of fluid pressure by way of said second spool to one or the other of said first spool pistons whereby said first spool may be reciprocated.

3. The invention of claim 2 further including cooperating detent means on said first spool and body whereby said first spool may be held in a plurality of positions. 7

4. The invention of claim 3 further including trigger valve means associated with said oscillation control valve whereby pressure fluid being communicated to either of said first spool pistons first passes through said trigger valve means where it is held until a preselected pressure is attained.

5. The invention of claim 4 wherein said trigger valve means comprises a pair of trigger valves and wherein each of said trigger valves comprises a body having a spool with a piston abutting one end and is biased by spring means at the other end.

6. The invention of claim 5 wherein the trigger valve additionally includes shims within said body and abutting said spring means whereby the spring means may be adjusted. 

1. In a scraper bowl having an apron pivotally mounted thereon, an apron tip pivotally mounted on the apron by an apron tip pivot means, hydraulic cylinder means for actuating the bowl, apron and tip, a source of fluid pressure, and main control valve means for directing fluid from the source of fluid pressure to the hydraulic cylinder means, the improvement comprising, oscillation control valve means for oscillating the apron tip in fore-and-aft arcuate motion about said apron tip pivot means, wherein the hydraulic cylinder means comprises a bowl cylinder, an apron cylinder, and a tip cylinder having head and rod ends, wherein the oscillation control valve means comprises a main control valve for actuating an oscillation control valve, and wherein the oscillation control valve comprises a body with a first and a second reciprocably mounted spool located therein, said first spool serving to communicate pressure fluid from said main control valve to either the head or the rod ends of the tip cylinder depending upon the position of said first spool, said second spool being biased to a first position by spring means at one end of said spool and being shiftable to a second position by means of a pressure fluid directed by said valve means for actuating said oscillation control valve, said second spool serving to direct pressure fluid to said first spool whereby said first spool is reciprocated thereby causing the tip cylinder to oscillate in a fore-and-aft manner.
 2. The invention of claim 1 wherein a first and second spool pistons are additionally mounted within said body and abutting each end of said first spool and wherein pressure fluid is alternatively directed from said source of fluid pressure by way of said second spool to one or the other of said first spool pistons whereby said first spool may be reciprocated.
 3. The invention of claim 2 further including cooperating detent means on said first spool and body whereby said first spool may be held in a plurality of positions.
 4. The invention of claim 3 further including trigger valve means associated with said oscillation control valve whereby pressure fluid being communicated to either of said first spool pistons first passes through said trigger valve means where it is held until a preselected pressure is attained.
 5. The invention of claim 4 wherein said trigger valve means comprises a pair of trigger valves and wherein each of said trigger valves comprises a body having a spool with a piston abutting one end and is biased by spring means at the other end.
 6. The inveNtion of claim 5 wherein the trigger valve additionally includes shims within said body and abutting said spring means whereby the spring means may be adjusted. 